Solid-state lighting arrays are used for a number of lighting applications. For example, solid-state lighting panels including arrays of solid-state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid-state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs), which may include inorganic LEDs, which may include semiconductor layers forming p-n junctions and/or organic LEDs (OLEDs), which may include organic light emission layers. Typically, a solid-state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region. A solid-state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs).
Cree, Inc. produces a variety of recessed downlights, such as the LR-6 and CR-6, which use LEDs for illumination. Solid-state lighting panels are also commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices, and for larger displays, such as LCD television displays.
Solid state lighting devices are typically powered with a DC signal. However, power is conventionally delivered in AC form. It is therefore generally desirable for a solid state light fixture to include an AC/DC converter to convert AC line voltage to a DC voltage.
Boost converters can be used to generate DC voltage from an AC line voltage with high power factor and low total harmonic distortion. The voltage of an LED-based load may be higher than the peak of the input (line) AC voltage. In that case, a single-stage boost converter can be employed as the driver, achieving high power efficiency and low cost. For example, a power factor corrected (PFC) boost converter which converts 120V ac, 60 Hz, to 200-250V dc output could be used to drive an array of high-voltage (HV) LEDs at a power level of 10-15 W.
For general lighting applications, it is desirable for a solid state lighting apparatus to be compatible with a phase-cut dimming signal. Phase-cut dimmers are commonly used to reduce input power to conventional incandescent lighting fixtures, which causes the fixtures to dim. Phase-cut dimmers only pass a portion of the input voltage waveform in each cycle. Thus, during a portion of a phase-cut AC input signal, no voltage is provided to the fixture.
Compatibility with phase cut dimming signals is also feasible for LED drivers based on boost converters. One low cost approach is to use open-loop control, which means a driver will not respond to the LED current decrease due to phase cut dimming, but rather keep the preset input current during dimmer conduction time. In this way, a ‘natural’ dimming performance is achieved, and input power, and thus LED current will reduce as the dimmer conduction time decreases. One other approach is for drivers to use closed-loop control. As control loops are complete and in effect, these drivers will try to compensate the input power decrease due to dimmer phase cut. In order to dim LEDs in these cases, the control loops should be saturated so that the input current cannot increase. The control loop saturation can be realized by clamping the output of an error amplifier, for example.
Another challenge due to phase cut dimming is for the boost converter to generate the bias voltage that is used to power the control circuits of the boost converter (so-called “housekeeping power”) during deep dimming. Conventional LED driver circuits draw housekeeping power from the auxiliary winding of a boost circuit inductor. Depending on how much of the input waveform is cut by the dimmer, during the dimmer off period, the control circuits can lose power. This can cause the entire driver circuit to cut off, which can result in visible flicker of the solid state lighting apparatus or other issues.